1. Field of the Invention
2. The Prior Art
Optical polarization beam splitter/combiners are known in the art. These devices may be used in optical communication in many ways including, but not limited to, optical power multipliers for combining two optical pump beams to increase optical pumping power, in coherent optical communications applications, and as optical polarization division multiplexers.
Two examples of prior art optical polarization beam splitter/combiners are shown in FIGS. 1A and 1B and FIGS. 2A and 2B, respectively. In the optical polarization beam splitter of FIG. 1A, an input beam enters from the fiber located at the left side of the figure. For the beam splitting operation, the incoming beam from fiber 1 at the left is collimated by the lens 1, and then enters the optical polarization beam splitter cube. The optical polarization beam splitter cube is able to split an arbitrary polarized light into two separated beams with orthogonally polarized directions. A first beam exits to the right and is coupled to fiber 2 through lens 2. A second beam exits in an upward direction and is coupled to fiber 3 through lens 3. In this way, an optical polarization beam splitter is realized. These three fibers could each be a single mode fiber, a polarization maintaining (PM) fiber, or even a multi-mode fiber.
The prior-art device depicted in FIG. 1A may be used as a bi-directional device if fiber 2 and fiber 3 are PM fibers, such that the optical beam polarization states coming from these two fibers are well defined and orthogonal to each other. Then these two beams can be added together at fiber 1 as shown in FIG. 1B. In this way, an optical polarization beam combiner is realized.
The device depicted in FIGS. 1A and 1B functions for its intended purpose. It however, suffers from several disadvantages, such as a larger device size necessitated by the need to employ an orthogonally disposed beam, suffer from a low extinction ratio, difficult to manage the fibers.
A second prior-art embodiment of an optical polarization beam combiner/splitter is depicted in FIGS. 2A and 2B. The optical principles of operation of the embodiment of FIGS. 2A and 2B are almost the same as those of FIGS. 1A and 1B.
Referring to FIG. 2A, a birefringent crystal is used to split the incoming arbitrary polarization light focussed from fiber 1 by lens 1 into two parallel beams having orthogonal polarization directions. These two beams are focused into fiber 2 and fiber 3, respectively, by lenses 2 and 3, such that the optical polarization beam splitter is realized. As shown in FIG. 2B, an optical polarization beam combiner can be realized as well. Compared to the embodiments of FIGS. 1A and 1B, the approach of FIGS. 2A and 2B provides high extinction ratio, employs fewer optical parts, and could be manufactured without the use of an optical epoxy in the optical path.
The device depicted in FIGS. 2A and 2B has its own drawbacks. Since fiber 2 and fiber 3 are located at the same side of the device, the birefringent crystal must have a length sufficient separate the two beams enough to accommodate the required spacing between lenses 2 and 3. Typically, lenses for this application have diameters of around 1.8 mm, requiring the minimum spacing between lens axes to also be about 1.8 mm. A birefringent must have a length of about 18 mm to provide the required beam separation of at least about 1.8 mm to accommodate the placement of lenses 2 and 3.
An optical polarization beam splitter according to one embodiment of the present invention comprises a first optical fiber having an end defining a first optical axis, a second optical fiber having an end defining a second optical axis, and a third optical fiber having an end defining a third optical axis parallel to and spaced apart from the second optical axis. A collimating lens is disposed along the first optical axis positioned to form a collimated optical beam from the first optical fiber. A focussing lens is disposed along a path of the collimated optical beam. A birefringent walk-off crystal has a first face adjacent to the focussing lens and a second face located at a focal plane of the focussing lens and in contact with the ends of the second and third optical fibers. The birefringent crystal is oriented such that and has a thickness between its first and second faces selected such that a first component of the optical beam having a first polarization exits the crystal at its second face and enters the end of the second optical fiber along the second optical axis and a second component of the optical beam having a second polarization orthogonal to the polarization of the first polarization exits the crystal at its second face and enters the end of the third optical fiber along the third optical axis.
An optical polarization beam splitter according to another embodiment of the present invention comprises a first optical fiber having an end defining a first optical axis, a second optical fiber having an end defining a second optical axis, and a third optical fiber having an end defining a third optical axis parallel to and spaced apart from the second optical axis. A collimating lens is disposed along the first optical axis and is positioned to form a collimated optical beam from the first optical fiber. A birefringent walk-off crystal is disposed in a path of the collimated optical beam. The crystal is oriented such that and has a thickness between first and second faces thereof selected such that a first component of the optical beam having a first polarization transits the crystal along a first path and a second component of the optical beam having a second polarization orthogonal to that of the first polarization transits the crystal along a second path disposed at a walkoff angle with respect to the first path. The first and second paths exit the second face of the crystal as substantially parallel first and second paths. A Wollaston prism (a pair of wedges) is disposed along the substantially parallel first and second paths and oriented such as to bend the substantially parallel first and second paths towards each other to form converging first and second paths. A focussing lens is disposed along the converging first and second paths and positioned such that a first component optical beam travelling along the first converging path is directed into the end of the second optical fiber along the second optical axis and a second component optical beam travelling along the second converging path is directed into the end of the third optical fiber along the third optical axis.
An optical polarization beam splitter according to another embodiment of the present invention comprises a first optical fiber having an end defining a first optical axis, a second optical fiber having an end defining a second optical axis, and a third optical fiber having an end defining a third optical axis parallel to and spaced apart from the second optical axis. A collimating lens is disposed along the first optical axis and is positioned to form a collimated optical beam from the first optical fiber. A first Wollaston prism is disposed in a path of the collimated optical beam and oriented such that a first component of the optical beam having a first polarization transits the prism along a first path disposed at a first angle with respect to the first optical axis and a second component of the optical beam having a second polarization orthogonal to that of the first polarization transits the prism along a second path disposed at a second angle with respect to the first optical axis, the first and second angles being substantially symmetrical about the first optical axis. A second Wollaston prism is disposed along the first and second paths and oriented such as to bend the first and second paths towards each other to form converging first and second paths. A focussing lens is disposed along the converging first and second paths and positioned such that a first component optical beam travelling along the first converging path is directed into the end of the second optical fiber along the second optical axis and a second component optical beam travelling along the second converging path is directed into the end of the third optical fiber along the third optical axis.
An optical polarization beam splitter according to another embodiment of the present invention comprises a first optical fiber having an end defining a first optical axis, a second optical fiber having an end defining a second optical axis, and a third optical fiber having an end defining a third optical axis parallel to and spaced apart from the second optical axis. The second and third optical axes are symmetrical about the first optical axis. A first focussing lens is disposed along the first optical axis. A Wollaston prism is disposed at a location along the first optical axis where a focal point of the first focussing lens lies on an interface between two component pieces of the prism wedge such that a first component of an optical beam from the first optical fiber having a first polarization is directed along a first path disposed at a first angle with respect to the first optical axis and a second component of the optical beam having a second polarization orthogonal to that of sa the id first polarization is directed along a second path disposed at a second angle with respect to the first optical axis, the first and second angles being substantially symmetrical about the first optical axis. A second focussing lens is disposed along the first and second paths at a location selected to focus a first component optical beam into the end of the second optical fiber to focus a second component optical beam into the end of the third optical fiber.